Earlier quoted context omitted.
To add on — the power you’re stealing from the ground is the power you can use for propulsion. So before you switch on the handrail skateboard, you have some power, P, available from the generator. You can use that P to power an air conditioner or a light bulb, or to crawl down the rail. The fact that you’ll then use that power for more propulsion is not relevant — Newton’s laws are linear so you can just add up the…
It’s gearing. Albeit elaborate. Intuitively what’s difficult to get the head around is sailing directly into the wind. However the penny dropped when somebody says it’s like sailing into the wind and that basically the propellers act like triangular sails that are continuously reorienting. The only tricky part bow is the “faster than wind” bit. Presumably this “dynamic sail” is better at harnessing any divergent ener…
The difference here -- you ARE moving, relative to the ground. You can, instead of coasting, reach down and grab some power from the huge mass rolling by below you. That'll try to slow you down -- it'll apply a force in the direction of the ground speed -- but if you harness some power and use it to drive a high efficiency propellor, you might be able to get a little extra speed, but as you speed up, you have more resistance both from the wind and the ground, so you can't do it forever.